"I thought the convict Barzan Ibrahim al-Tikriti had escaped the noose. I shouted that he's escaped the noose, go down and look for him. I went down a few steps ahead of the others to see: I found out that his head had separated from his body."

The executions took place at 0300 (0000 GMT), apparently in the same building where Saddam Hussein was put to death on 30 December after being convicted of the same crime.

The manner of his execution has sparked controversy around the world, after unofficial mobile phone footage was released showing him being taunted and insulted in his final moments.

Iraq's Shia-dominated government pledged a full investigation. Government spokesman Ali al-Dabbagh said this time everyone present at the facility had signed a document pledging appropriate behaviour.

Correspondents say the gruesome detail about Barzan's decapitation was probably made public in order to avoid it being leaked later with accompanying allegations of mistreatment.

A member of their defence team, Issam al-Ghazzawi, told the Reuters news agency he was outraged by the execution.

"When a man is hanged, he does not lose his head," he said. "The way Barzan was executed is shameful."

The bodies of the men are to be handed over to their families within the next few days.

'Key target

Barzan Ibrahim al-Tikriti was the former Iraqi leader's half-brother and served as the head of his feared secret police, the Mukhabarat.